Nanocomposites with different metals as magnetically separable nanocatalysts for oxidation of aldehydes

Article abstract of DOI:10.1016/j.crci.2016.02.009

In this study, two metals were chosen for composing two different nanocatalysts. Zinc acetate and nickel chloride were used to prepare two nanocatalysts from acetanilide anchored to functionalized silica-coated Fe₃O₄ magnetic nanoparticles. They were identified using scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and gas chromatography–mass spectrometry. These nanocatalysts were used for the oxidation of the following aldehydes: 3-hydroxybenzaldehyde, 4-methoxybenzaldehyde, and 3-nitrobenzaldehyde. High efficiency, stability, recoverability, recyclability, and selectivity were achieved using these nanocatalysts.

Full text of DOI:10.1016/j.crci.2016.02.009

C. R. Chimie xxx (2016) 1e6
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Full paper/M eꢀ moire
Nanocomposites with different metals as magnetically
separable nanocatalysts for oxidation of aldehydes
*
Akbar Esmaeili , Sahar Kakavand
Department of Chemical Engineering, North Tehran Branch, Islamic Azad University, PO Box 19585/936, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, two metals were chosen for composing two different nanocatalysts. Zinc
acetate and nickel chloride were used to prepare two nanocatalysts from acetanilide
anchored to functionalized silica-coated Fe O magnetic nanoparticles. They were iden-
3 4
tified using scanning electron microscopy, X-ray diffraction, Fourier transform infrared
spectroscopy, and gas chromatographyemass spectrometry. These nanocatalysts were
used for the oxidation of the following aldehydes: 3-hydroxybenzaldehyde, 4-
methoxybenzaldehyde, and 3-nitrobenzaldehyde. High efficiency, stability, recoverability,
recyclability, and selectivity were achieved using these nanocatalysts.
Received 12 January 2016
Accepted 18 February 2016
Available online xxxx
Keywords:
Magnetic nanocatalyst
Acetanilide
Nickel chloride
Aldehyde
©
2016 Acad eꢀ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
problem with separation of the catalyst from the reaction
mixture. A proper technique for facilitating separation is
Catalysts have a great effect on chemical reactions
magnetic separation [2]. Recently, magnetic nanoparticles
(MNPs) have been broadly used as catalyst supports because
of their highly active surface, which causes high loading
capacity of the catalyst, high dispersion, remarkable stabil-
ity, and ease of recovery [7]. Moreover, magnetic particles
can withstand virtually all except highly acidic chemical
environments. Because MNPs are highly susceptible to
oxidation and agglomeration, they are best configured as a
coreeshell structure making use of either organic or inor-
ganic shells [8]. Among different coatings, silica supports
have the advantage of stability and inertness, facilitating
functionalization, low cost, and high surface area [9]. The
method of using metaleligand grafting on silica-coated
magnetite leads to higher loading than straight grafting of
metals onto the same type of surface. Moreover, amine-
functionalized nanoparticles have more catalytic activity
than metal catalysts with silica coating. Among different
metals, nickel and especially zinc are good choices because
of their Lewis acid property, which helps the oxidation re-
action [10]. Conventionally, oxidation has been performed
without any catalyst by using an oxidant alongside mineral
acids. This procedure is, however, harmful because it pro-
duces high amounts of dangerous wastes. To reduce this
because they enhance the efficiency of reactions and
decrease process temperatures [1]. Homogeneous catalyst
particles dissolve easily in a reaction mixture, whereas
heterogeneous catalyst particles do not [2]. High activity and
good selectivity are benefits of homogeneous catalysts [3],
as opposed to the restricted activity of heterogeneous cat-
alysts. A major difficulty in processing of homogeneous
catalysts is that after the reaction has completed, separation
of the dissolved catalyst from the final mixture is difficult
[
4]. Unlike homogeneous catalysts, heterogeneous catalysts
separate easily from the reaction mixture and thus do not
cause product impurity [5]. Nanoparticles can be used to
exploit both the high surface activity of homogeneous cat-
alysts and the capability of separation of the catalyst at the
end of the reaction found in heterogeneous catalysts [1].
Nanoparticles, especially metallic and metal oxide nano-
particles, are the most effective nanostructures for sup-
porting catalysts [6]. Nanoparticles do not present a
problem in recycling the used catalyst, but they do present a
*
Corresponding author.
E-mail addresses: akbaresmaeili@yahoo.com, akbaresmaeili@iau.ac.ir
A. Esmaeili).
(
http://dx.doi.org/10.1016/j.crci.2016.02.009
631-0748/© 2016 Acad eꢀ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1
Please cite this article in press as: A. Esmaeili, S. Kakavand, Nanocomposites with different metals as magnetically separable
nanocatalysts for oxidation of aldehydes, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.02.009

2
A. Esmaeili, S. Kakavand / C. R. Chimie xxx (2016) 1e6
hazard to the environment, scientists have attempted to
discover and enhance reactions that follow the doctrines of
green chemistry [11]. The oxidation of aldehydes is a widely
used reaction in organic chemistry [12]. Following the pro-
cedure used by Sharma and Monga [13], magnetite nano-
particles were coated with silica and then functionalized
with (3-aminopropyl)triethoxysilane (APTES). Acetanilide
was introduced to these particles, and finally zinc acetate
and nickel chloride were used to prepare two different
nanocatalysts. These were used for oxidation of aldehydes.
Their easy separation from the reaction mixture was
possible because of their magnetic characteristic [6].
High efficiency of the catalyst (with Zn as the metal) was
exhibited. Significant conversion of different aldehydes to
their acids was yielded (4-methoxybenzoic acid 98.3%, 3-
nitrobenzoic acid 97.5%, and 3-hydroxybenzoic acid 97.2%)
during the reactions performed under optimal conditions,
even after six runs the magnetic nanocatalyst remained
recyclable.
were recorded using a Thermo Scientific Nicolet 8700 FT-IR
spectrometer. The product of oxidation was analyzed and
validated using gas chromatographyemass spectrometry
(GCeMS).
2.3. Amino-functionalized SiO -coated Fe O nanoparticle
2
3 4
preparation
We prepared MNPs using a coprecipitation approach. To
this end, we dissolved 6.0 g of ferric sulfate and 4.2 g of
ferrous chloride in 250 mL of water and stirred the mixture
ꢀ
at 60 C to obtain an orange solution. Next, we subjoined
15 mL of ammonium hydroxide (25%) by stirring, which
turned the color of the solution to black, and continued
stirring for 30 min. We separated the MNPs and washed
them with deionized water and ethanol. We carried out
coating of the MNPs by the solegel method. We sonicated
the solution of activated MNPs in 0.5 g of HCl (0.1 M)
combined with 200 mL of ethanol and 50 mL of water, and
then added 5 mL of ammonium hydroxide (25%) and 1 mL
of tetraethyl orthosilicate to the suspension. We continued
2
. Materials and methods
ꢀ
stirring at a temperature of 60 C for 6 h. We separated the
2
.1. Materials
resulting SiO -coated MNPs (SMNPs) and washed them
2
with ethanol. Finally, we sonicated 1.0 g of SMNPs in
APTES and tetraethyl orthosilicate were purchased from
700 mL of ethanol and charged the flask containing
dispersed SMNPs with 5 mL of APTES, refluxing for 6 h at
80 C to produce amino-functionalized SMNPs (ASMNPs).
Separation and washing of the ASMNPs with ethanol were
performed to remove the unreacted silylating agent.
Merck, Germany. Hydrochloric acid (36.5%), acetanilide,
zinc acetate, nickel chloride, ethanol, acetone, dichloro-
methane, acetonitrile, and hydrogen peroxide were ob-
tained from Research Laboratory.
ꢀ
2
.2. Characterizations
2.4. Synthesis of Zn(II)eacetanilide complex anchored to
ASMNPs
Perusal of nanocatalyst morphology was performed by
scanning electron microscopy (SEM) on a KYKY-EM3200
scanning electron microscope. The phases of the products
were assayed by X-ray diffraction (XRD) on a powder
To anchor acetanilide onto the ASMNPs, we refluxed
3.0 g of ASMNPs and 6.0 mmol of acetanilide in 375 mL of
ꢀ
ethanol at 80 C for 3 h. To synthesize the nanocatalyst, we
diffractometer using Cu K
a
radiation (
l
¼ 1.54060 Å) in the
stirred 2.0 g of anchored ASMNPs in a solution of 8.0 mmol
of zinc acetate in acetone for 4 h. Finally, we separated the
2
q
interval. Fourier transform infrared spectroscopy (FT-IR)
Scheme 1. Route for the synthesis of magnetic nanocatalyst with Ni as the metal.
Please cite this article in press as: A. Esmaeili, S. Kakavand, Nanocomposites with different metals as magnetically separable
nanocatalysts for oxidation of aldehydes, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.02.009

A. Esmaeili, S. Kakavand / C. R. Chimie xxx (2016) 1e6
3
and extracted using dichloromethane and 10% sodium bi-
carbonate. The organic layer was dried with sodium sulfate
ꢀ
and the solvent removed under vacuum at 66 C to obtain
the final product. GCeMS was used to validate the structure
of the product.
3. Results and discussion
2 2 3 4
3.1. Characterization of Zn(II)eacetanilide@NH eSiO eFe O
and Ni(II)eacetanilide@NH eSiO eFe
2
2
3 4
O
The SEM image in Fig. 1 shows the monotonic scattering
of zinc complex upon the surface of the silica-coated
magnetite nanoparticles and is further documented by
comparison of the SEM image gained in this study with a
previous study [13]. Measurements of the crystalline
structures for MNPs and ASMNPs were performed by pow-
der XRD (Fig. 2), which shows the peaks of a cubic inverse
spinel structure. The spacings and relative intensities in the
diffractogram are consistent with the Joint Committee on
Powder Diffraction Standards XRD data (Card No.19-06291)
for Fe O . Previous study has validated coating of MNPs for
Fig. 1. SEM image of a nanocatalyst with Zn as the metal.
obtained Zn(II)eacetanilide@ASMNP nanocatalyst and
washed it completely with water.
2
.5. Synthesis of Ni(II)eacetanilide complex anchored to
ASMNPs
3
4
widening of XRD peaks of SMNPs [14]. In our study, syn-
thesis of MNPs, SMNPs, and ASMNPs is evident.
The above-mentioned procedure was repeated to pre-
ꢁ
1
pare the Ni(II)eacetanilide complex nanocatalyst, with
only two differences: nickel chloride was used instead of
zinc acetate and methanol instead of acetone (Scheme 1).
In FT-IR studies (Fig. 3), a peak at 1632 cm in Fig. 3d
indicates anchoring of acetanilide as ligand on
NH eSieFe O . This peak is characteristic of the C]N
a
2
3 4
ꢁ1
stretching vibration. The absorption peak at 1636 cm in
Fig. 3e corresponds to binding of zinc as a metal to the
ligand, which is confirmed in comparison with previous
research [15].
2
.6. Evaluation of catalyst activity in oxidation of aldehydes
We poured 10 mmol of benzaldehyde and 200 mg of a
nanocatalyst into 10 mL of acetonitrile and refluxed the
mixture at 80 C for 3 h, adding 1.5 mL hydrogen peroxide
Fig. 4 displays the magnetization curves of the MNPs
[16] and the two magnetic nanocatalysts. They exhibited no
hysteresis at room temperature. The saturation magneti-
zation of three samples was 60, 30, and 18 emu/g, respec-
tively. In comparing the vibrating sample magnetometry of
magnetic nanocatalysts with Zn as the metal in this study
ꢀ
(
30%) dropwise to the reaction mixture during the reflux.
After 3 h, we allowed the reaction mixture to cool and
aggregated the nanocatalyst using a magnet at the side of
the flask. The residual solution was collected with a pipette
Fig. 2. XRD pattern of (a) MNPs and (b) ASMNPs.
Please cite this article in press as: A. Esmaeili, S. Kakavand, Nanocomposites with different metals as magnetically separable
nanocatalysts for oxidation of aldehydes, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.02.009

4
A. Esmaeili, S. Kakavand / C. R. Chimie xxx (2016) 1e6
used for oxidation of benzaldehyde. Although both nano-
catalysts transmuted benzaldehyde into benzoic acid with
high efficiency, the zinc nanocatalyst had higher trans-
formation (100%) than the nickel nanocatalyst. The optimal
time and temperature for conversion were found to be 3 h
ꢀ
and 80 C, respectively.
As these nanocatalysts retained their high catalytic ef-
ficiency (100% for the nanocatalyst with zinc and 55% for
the nanocatalyst with nickel) after six cycles, their reus-
ability is confirmed. Different time periods (1.5, 3, and 6 h)
were tested on the zinc nanocatalyst. The highest rate of
transformation (100%) was obtained at 3 h. When the time
was decreased to 1.5 h, transformation decreased to 65%.
When the time was increased to 6 h, transformation
decreased to 36% (Fig. 5). For these time experiments, the
ꢀ
temperature was set at 80 C.
The nanocatalyst with zinc was used for the oxidation of
different aldehydes. Oxidation of 3-hydroxybenzaldehyde
and 3-nitrobenzaldehyde was carried out under the same
conditions. Acetonitrile was used as a solvent (10 mL), H
2 2
O
(
1.5 mL) as an oxidant, and dichloromethane was used for
Fig. 3. FT-IR spectra of (a) MNPs, (b) SMNPs, (c) ASMNPs, (d) Zn@ASMNPs,
and (e) acetanilideeZn@ASMNPs.
extraction of the organic phase. The reaction was continued
for 3 h at 80 C. For 4-methoxybenzaldehyde, the reaction
ꢀ
was continued for 6 h. GCeMS was used for evaluation of
the conversion. The highest conversion was obtained for
the oxidation of 3-hydroxybenzaldehyde, with oxidations
of 4-methoxybenzaldehyde and 3-nitrobenzaldehyde hav-
ing lower conversions (Fig. 6).
with previous research [13], higher magnetization of our
catalyst is obvious; the other nanocatalyst with Ni as the
metal (which to the best of our knowledge has not been
synthesized before) had equal magnetization to the Zn
magnetic nanocatalyst in the previous study [13]. The
decreased saturation magnetization seen in the nano-
catalysts compared with the MNPs is because of the exis-
3.3. History of nanocatalysts used for the oxidation of
2
tence of nonmagnetic elements (such as SiO , Zn/Ni, and
aldehydes
acetanilide) in the nanocatalysts. Despite this reduction in
saturation magnetization, these particles were separated
from the reaction medium with an external magnet.
Different nanocatalysts have been used for the oxidation
of aldehydes. Of note is the use of Fe nanoparticles
3 4
O
activated by ethyl acetoacetate for the oxidation of different
aldehydes in solvent-free conditions and comparing two
3
.2. Catalytic efficiency
oxidants: H
version was achieved with tert-butyl hydroperoxide [17].
2 3
Another study used biosilica-supported Fe O
2 2
O and tert-butyl hydroperoxide; higher con-
2
The prepared nanocatalysts Zn(II)eacetanilide@NH e
SiO
2
eFe
3
O
4
and Ni(II)eacetanilide@NH
2
eSiO
2
eFe
3
O
4
were
Fig. 4. Magnetization curves of (a) MNPs, (b) acetanilideeZn@ASMNPs, and (c) acetanilideeNi@ASMNPs.
Please cite this article in press as: A. Esmaeili, S. Kakavand, Nanocomposites with different metals as magnetically separable
nanocatalysts for oxidation of aldehydes, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.02.009

A. Esmaeili, S. Kakavand / C. R. Chimie xxx (2016) 1e6
5
Fig. 5. GCeMS results for the oxidation product of benzaldehyde by a nanocatalyst (a) with Zn after 3 h and (b) after 1.5 h and (c) with Ni after 3 h and (d) with Zn
after 6 h.
nanoparticles as a nanocatalyst, using H
2
O
2
as an oxidant
efficiency of each nanocatalyst was evaluated. High trans-
formations (100% for the nanocatalyst with zinc and 55% for
the nanocatalyst with nickel) were obtained under the
and acetonitrile as the solvent [18]. Other research used an
ironeoxalate capped ironecopper bimetallic oxide nano-
material for the oxidation of aldehydes to their acids and
esters [19]. Other catalysts investigated for this purpose
include bio-derived CuO nanoparticles [20], oxoammo-
nium salt, Mohr's salt [21], and gold [22].
2 2
optimal conditions of 0.2 g of a nanocatalyst,1.5 mL of H O ,
10 mL of acetonitrile, and 1 mL of substrate (benzaldehyde)
ꢀ
at 80 C for 3 h. The catalyst could be isolated from the
reaction medium with an external magnet. The nano-
catalysts were used for six runs without losing their ac-
tivity, which demonstrated their efficiency, recoverability,
and reusability. The use of zinc as a metal especially
increased the efficiency of the nanocatalyst because of its
Lewis acid property. These features make these nano-
catalysts appropriate heterogeneous catalysts for oxidation
of aldehydes.
4
. Conclusions
Two heterogeneous catalysts based on different metals,
that is, Ni(II)eacetanilide@NH
acetanilide@NH eSiO eFe , were prepared and used for
the oxidation of benzaldehyde to benzoic acid. The catalytic
2 2 3 4
eSiO eFe O and Zn(II)e
2
2
3 4
O
Fig. 6. GCeMS results for the oxidation product of (a) 3-hydroxylbenzaldehyde, (b) 4-methoxybenzaldehyde, and (c) 3-nitrobenzaldehyde by a nanocatalyst with
Zn as the metal.
Please cite this article in press as: A. Esmaeili, S. Kakavand, Nanocomposites with different metals as magnetically separable
nanocatalysts for oxidation of aldehydes, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.02.009

6
A. Esmaeili, S. Kakavand / C. R. Chimie xxx (2016) 1e6
[11] D. Astruc, Nanoparticles and Catalysis, John Wiley & Sons, Wein-
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2
Please cite this article in press as: A. Esmaeili, S. Kakavand, Nanocomposites with different metals as magnetically separable
nanocatalysts for oxidation of aldehydes, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.02.009